Improve coker efficiency with reliable valve automation

The case history discusses conditions occurring in delayed coker operations at two refineries in Louisiana, the impacts on valve performance, and a solution that offers extended life, better reliability and other benefits.

A refinerys delayed coker unit (Fig.
1) operates under what are perhaps the harshest
conditions of any process in the plant. Equipment with moving
parts, notably valves and the actuators that operate them
(Fig. 2), are especially vulnerable to these
severities. The following case history discusses conditions
occurring in delayed coker operations at two refineries in
Louisiana, US; the impacts on valve performance; and a solution
that provides extended life, increased reliability and other benefits at
the facilities.

Fig.
1. Multiple valves and actuators control
the coking/decoking process.

Fig. 2. Actuators are
compact and versatile so as to be configured in a
variety of mounting configurations.

Severe conditions in the coker

Severe operating conditions, including excessive heat,
vibration and corrosion, exist in every refinerys delayed coker
operation. High inlet temperatures of the residual oil flowing
from the fractionator through the transfer line into the coke
drum exceed 800°F (425°C) at low pressures of 10 psig
to 15 psig.

As the operating drum fills with coke, torques on the
valves wetted parts tend to increase, putting additional
stress on the transfer line ball valve and added operational
torque on the multi-turn electric valve actuators. During the
coke removal process, there is extreme vibration. The
high-pressure water lines used to drill out and cut the coke
from the drum internals create pressures of up to 4,000 psi.
The steam and quench water piping used in the decoking process
is susceptible to rapid expansion and temperature
fluctuations of condensate and/or water, producing an often
violent water hammer effect. Corrosion is present as the
cokes traces of sulfur combine with the units
washdown water.

During the washdown phase, a significant amount of abrasive,
airborne dust is created, covering all surfaces within the drum
and its immediate surroundings. The coke dust not only creates
challenges for corrosion protection, but also builds up in
crevices, impeding instrument and equipment functionality. In
addition to the harsh conditions present, the space available
for performing maintenance is confined, hot and
potentially dangerous for plant personnel.

Impact on valves and actuators

Typically, there are eight to 10 valves for each drum in the
coking process. These valves perform multiple services
including recirculation, switching, quenching, washdown, steam
hydrocarbon stripping and drum steam
reheating. They control flow in piping that transports steam,
water, slurry, hydrocarbons and product, and they are critical
to the operation.

Controlled by a programmable logic command, the valves
sequential event must be consistently and reliably executed by
an actuator (Fig. 3). To maintain process
control, valves have safety interlocks restricting their
opening and closing through limit switches. An inoperable valve
actuator must be reinstated quickly so that the system can
continue functioning. If actuators fail, the valves must be
opened or closed manually. This is a strenuous, time-consuming
and potentially dangerous process for the unit operator,
although it is necessary to keep the coking/decoking process in
sequence and on schedule.

Fig. 3. An actuator executes
sequential
events for coker valves.

Automated valve performance and mean time between failure
(MTBF) are constant and costly problems for the refinery. While valves are not as
susceptible to failure and can generally be serviced during
periodically scheduled shutdowns, actuators have been prone to
early, unplanned failures and the need for continual
repair.

Coker actuator failure can occur almost immediately after
installation, and normal life expectancy is less than one year.
Experience has shown that actuator failure can be attributed to
a variety of conditions that are fairly consistent in any coker
application. Water hammer and vibration effects can break
internal electrical connections and dislodge sensitive
microprocessor components. Also, motors can become disconnected
from the actuator housing through vibration. When inspected,
these motors have been found precariously hanging from their
wiresand nothing else.

Maintenance costs can be excessive,
accumulating with continual callout and overtime repair
charges. Ongoing actuator problems have even necessitated the
use of a full-time, dedicated troubleshooter whose sole
function is to keep the valves operating as scheduled, or
repaired in a timely manner.

A successful solution

In the case of two Louisiana refineries with delayed coker
operations, the search for a robust and reliable actuator
solution to provide extended service life resulted in the
testing and selection of a modified, multi-turn, electric
design that had provided exceptional service in other
applications.

While not the newest version of the technology, this established
actuator had been engineered to provide the necessary features
for success in this challenging service. The selected unit used
industrial-grade epoxy coatings on all external surfaces as
standard construction to provide excellent
corrosion resistance. Also standard was its marine-grade
aluminum enclosure with stainless-steel captured bolting.

Unlike some newer and more sophisticated actuator designs,
this solution had no microprocessor components. Rather, it
featured incorporated, reliable circuit boards with no
termination wiring, and compact internal limit switches and
relays that could withstand the high temperatures in a delayed
coking unit.

The actuator could be remotely managed from a hardwired
push-button panel unaffected by the dust; yet, the control was
within sight of the actuator to verify its proper operation.
Internal control components were smaller, compact and
lightweight to resist self-destruction from inertia and the
momentum generated from vibration and water hammer effects. Its
linear drive train and gearbox assembly were specifically
configured to withstand high torque and thrust loads while
maintaining alignment.

Sizing safety factors were also considered. For reliable
operation, this actuator was sized to provide a two-times
safety factor, which is highly recommended to account for the
variable and generally higher torques needed as the process
progresses and as piping expands and contracts, requiring
higher torques not included in new torque values. The overall
actuator footprint and weight were sufficient for operation in
confined spaces and for accommodating a variety of horizontal
and vertical valve installation positions.

After being tested and installed, the selected actuator
provided the refineries with immediate relief from failure. The
units have been in continuous service for more than six years
in one refinery and for more than three years in the other.
There have been virtually no failures, and the limited need for
maintenance can be completed during scheduled plant
shutdowns.

Takeaway

The importance of valves and the electric actuators that
control the operation in a delayed coker unit cannot be
minimized. As a batch process, the coker becomes the potential
bottleneck in an otherwise continuous refining operation. If the critical
coke drum filling, drum switching and decoking schedule is
significantly interrupted, it can impact the entire refinery
throughput, costing the company millions of dollars per day in
lost production.

The valve actuator, seemingly a very small item in the total
process, has a significant importance and a great impact on the
delayed coking operation. Premature failure can lead to extra
costs for operator overtime, additional labor and safety risks
for manual valve operation, replacement costs and potential refinery downtime.

The robust, reliable solution was found to be a
field-proven, ductile iron-housed, powder-coated actuator with
external control in proximity of the valve. Its compact
physical size and weight, unique internal electric circuitry
configuration and modified hardwired operation have provided
the refineries with considerable cost savings and have averted
potential production outages.

The refineries using the selected actuators have been able
to increase production, improve personnel safety and conduct
regularly scheduled shutdowns, thereby significantly reducing
maintenance costs.
HP

The authorsBenny Deters is the director of
manufacturing at Calabrian Corp., a specialty
chemical manufacturer in Port Neches, Texas. He is an
electrical engineering graduate from McNeese State
University, and he has more than 25 years of
management experience in delayed coking and plant
operations.
Ross Wolkart is the gas and
pneumatic product manager for Emerson Process
ManagementValve Automations EIM line. He
is the companys refining and pipeline
application specialist and a graduate of Southeastern
Louisiana University. Mr. Wolkart has more than 16
years of sales and service experience and expertise
in delayed coking operations.

Delayed coker facts

 What is delayed coking? A semi-batch thermal
cracking process used to upgrade and recover residual
liquid and gas streams
while leaving behind petroleum
coke, which is usable as an industrial fuel source or
can be further
refined for additional industrial
purposes

Certain MOV's have experienced 'ghost' movement with no operator action and proper interlock status. Such movement can be prevented by de-energizing the actuator when not in use. Designs which eliminate this possibility are of benefit due to the vibration on the structure. A follow-up article could address these types of events.

V K Kapoor04.28.2013

It is a nice and reliable solution for the tough problems that valves and actuators have to face to be rugged in Delayed Coker

SHRI K GOYAL04.26.2013

The article is good for the industry. Valves and actuatrs have been a big issue on cokers. These small, but very important improvements help a lot in the long run.Small suggestion - facts about coker should be reviewed.Shri Goyal